Hydraulic apparatus

ABSTRACT

A hydraulic control system comprising a master piston-andcylinder unit adjustable by a main control member to displace liquid, a plurality of slave piston-and-cylinder units hydraulically connected to the master cylinder to displace liquid and intended for connection to a plurality of subsidiary control members, and mechanical means such as springs, operative as liquid is displaced from the master piston-and-cylinder unit to cause movement of the slave units in a desired sequence. The control system may be combined with a hydraulic power transmission comprising a fixed positive displacement pump and a variable positive displacement motor, a first slave piston-andcylinder unit being arranged to adjust motor displacement and a second slave piston-and-cylinder unit being arranged to control a variable by-pass for selectively by-passing a proportion of pump delivery such that continuous adjustment of the main control member will sequentially adjust the variable by-pass and motor displacement.

Waters [45.1 Mar. 19, 1974 HYDRAULIC APPARATUS Primary Examiner-Edgar Geoghegan Assistant Examiner-A. M. Zupcic 75 inventor: John Henr Waters Cheltenham, 1 England y Attorney, Agent, or FirmYoung & Thompson [73] Assignee: Dowty Hydraulic Units Limited, 57 ABSTRACT Arley Court, Cheltenham, England 1 A hydraulic control systemv comprising a master [22] Flled: May 241 1971 piston-and-cylinder unit adjustable by a main control [21] APPL NOJ 146,105 member to displace liquid, a plurality of slave piston- 30 F A P D t and-cylinder units hydraulically connected to the masorelgn pplca y a a ter cylinder to displace liquid and intended for con- May 26, 1970 Great Britain ..25l56/70 nectionrto a plurality f Subsidiary 1 members [52] US. Cl. 60754 3 and mechanical means Such as Springs Operative as 51 Int. Cl. Fl5b /18 liquid is displaced fmm the master pismn'and'cylinder Field of Search U /53 R 5 VS 427 444 unit to cause movement Of the slave units in a desired 60/443 489 sequence. The control system may be combined with a hydraulic power transmission comprising a fixed posi- [56] References cued tive displacement pump and a variable positive displacement motor, a first slave piston-and-cylinder unit UNITED STATES PATENTS being arranged to adjust motor displacement and 21 3,533,234 10/1970 Futamata o0/444 second Slavev piswn and cy1inder unit being arranged g 28 22: to control a variable by-pass for selectively bypassing gagi 51 :23 $444 a proportion of pump delivery such that continuous L 'Q 6/1965 VS adjustment of the main control member will sequen- 1116111559 1/1 112 Worn 60/53 R tially adjust the variable y-P and motor displace- 3,475,9(J3 11/1969 Christenson 60/52 VS 15 Claims, 2 Drawing Figures MOTOR 738 H lu-' 1; A 7

PATENIEDHAR 19 I974 3797'. 244

SHEET 1 BF 2 ATTORNEYS PATENTEDHAR 1 9 m4 SHEET 2 BF 2 PUMP INVENTOR ATTORNEYS HYDRAULIC APPARATUS This invention relates to a hydraulic control system to effect sequential movements of a number of control members. It is normal where sequential control for a number of control members is required to provide a mechanical cam device operative on the control members in the required sequence. However, a cam or equivalent purely mechanical sequential control has the disadvantage of requiring particular positional relationships of the control members relative to the cam or the like. The main object of the present invention is to provide a single main control member capable during continuous movement of effecting sequential operation of a number of subsidiary control members, the subsidiary control members being locatable in their operative positions independently of one another.

In accordance with the present invention, a hydraulic control system comprises, a master piston-and-cylinder unit adjustable by a main control member to displace liquid, a plurality of slave piston-and-cylinder units hydraulically connected to the master cylinder to receive displaced liquid and intended for connection to a plurality of subsidiary control members, and mechanical means operative as liquid is displaced from the master piston-and-cylinder unit to cause movement of the slave units in a desired sequence.

The mechanical means may comprise loading springs for the slave units arranged to require differing pressures and/or pressure ranges for movement of some or all of the slave units, and the main control member is adapted to exert successively increasing forces on the master unit to displace liquid at increasing pressures to effect the sequential operation of the slave units.

The sequence may be modified by providing means for causing movement of a slave unit independently of pressure liquid from the master unit. During such modified movement the liquid may flow between slave units causing relative adjustment thereof without requiring movement of the master unit.

One slave unit may incorporate two or more stages of spring loading whereby the slave unit movement takes place in stages, each in response to the corresponding pressure range of liquid displaced by the master cylinder.

The hydraulic control system set out above may be used in combination with a hydraulic power transmission comprising, a fixed positive-displacement pump and a variable-positive-displacement motor, wherein a first slave piston-and-cylinder unit is arranged to adjust motor displacement and a second slave piston-andcylinder unit is arranged to control a variable by-pass for selectively by-passing a proportion of pump delivery to control the flow rate of liquid to the motor.

One embodiment of the invention will now be particularly described with reference to the accompanying diagrammatic drawing FIGS. 1 and 2 of which together illustrate a hydraulic control system. This embodiment is intended for use as a hydraulic power transmission.

Power for the transmission is supplied by an engine 1 of any conventional type which may be either adjusted for running at constant speed, or may be independently adjustable for speed. The transmission comprises a fixed positive-displacement pump 2 for example of the gear type, hydraulically connected to drive a variable-displacement motor 3 which in turn is mechanically connected to drive a load. The motor 3 may be of the swash plate type whose displacement is adjustable by movement of a plunger 4 relative to the motor. As shown, outward movement of the plunger 4 increases the motor displacement to a maximum. The pump 2 draws liquid from a low pressure reservoir 5 and delivers liquid at pressure to a pipe 6 which passes to a flow dividing valve 7. The valve 7 comprises a valve spool 8 slidable within a cylinder 9 having three ports 11, 12 and 13 formed therein. The pipe 6 connects to the port 12. The spool member 8 includes three lands 14,15 and 16 of which the central land 15 is somewhat narrower than the port 12 and by variation in position is adapted to divide the flow from port 12 between the ports 11 and 13. The lands 14 and 16 define a pair of working spaces 17 and 18 at the two ends of the cylinder 9. The working space 18 is connected by an internal passage 19 within the spool member to receive the pressure within the port 11. Working space 17 contains a spring 21 and receives liquid at pressure from a pipe 22 through a restrictor 23. A pipe 24 carries liquid from port 11 to a variable throttle valve 25. The port 13 is connected to a by-pass passage 26 leading to reservoir 5.

The variable throttle valve comprises a cylinder 27 having five ports 28, 29, 31., 32 and 33 formed therein. A spool valve member 34 slides in the cylinder 27 and has three lands 35, 36 and 37 connected by tapered portions 38 and 39. The lands and 37 define working spaces 41 and 42 at the two ends of cylinder 27. The working space 42 in addition to receiving hydraulic liquid at pressure also contains a pair of loading springs 43 and 44. The loading spring 43 is retained on the spool member 34 so as to permit a small degree of movement thereof before spring load in either direction is applied from the spring 43. The spool 34 is locatable in either of the limits of free movement by means of a pawl 45 engaging a flange 46 on the spool 34. The spring 44 is so arranged, after the spool 34 has moved a predetermined distance in either direction when compressing spring 43, to add a further spring load resisting further movement of spool 34. The force required to move the spool 34 is effected by hydraulic pressures fed to the working spaces 41 and 42.

The hydraulic motor 3 is connected by pipes 47 and 48 to the ports 29 and 32 so that depending on the position of spool 34 flow from pipe 24 may pass to either of the two pipes 47 or 48. The return flow from the motor which flows along either pipe 47 or 48 will, depending on the position, of spool 34, enter either of the ports 28 or 33. These ports are connected together by a pipe 49 for connection to a braking valve 51. A pair of auxiliary ports 52 and 53 open into the cylinder 37 on either side of the port 31 and are alternatively closable by the land 36. The ports 52 and 53 are connected together externally of the throttle valve 25 to the pipe 22 which is connected to various valves within the circuit. In particular, the pipe 22 connects through restrictor 23 to the working space 21.

The braking valve 51 comprises a cylinder 54 having a pair of ports 55 and 56 formed therein and a spool 57 slidable in the cylinder. The spool 57 includes a pair of lands 58 and 59 which control the flow between the ports 55 and 56 and which also define a pair of working spaces 61 and 62 at the ends of the cylinder 54. The working space 62 includes a spring 63 acting on the spool. The working space 61 is fed with liquid by a re stricted passage 64 within the spool. Working space 62 is fed with liquid by a restricted passage 65 within the spool.

Pressure control within working space 62 is effected both by a pilot valve 66 and a pilot valve 67. The pilot valve 66 is a spring loaded valve and responds to bydraulic pressure from the pipe 49 to connect working space 62 to drain through pipes 68 and 69. The pilot valve 66 will connect working space 62 to drain when pressure in pipe 49 approaches 2000 p.s.i. The pilot valve 67 will connect working space 62 to drain through pipes 71 and 72 in response to pressures in pipe 22 above 250 p.s.i. The reduction of pressure in working space 62 by operation of either of the pilot valves 66 or 67 opens a connection in valve 51 between ports 55 and 56 permitting return flow liquid from the motor from port 28 or 33 to pass through to port 56.

A boost pressure valve 73 comprises a cylinder 74 having three ports 75, 76 and 77 formed therein and a spool 78 slidable therein. The spool 78 includes two lands 79 and 81 which control the connection between port 76 to either of the ports 75 or 77. The spool 78 is acted upon by a spring 82 to urge it against the force in a small working space 83 formed by an auxiliary piston and cylinder and connected to the pipe 22. The port 75 receives liquid from pipe 6 at pump delivery pressure whilst the port 77 is connected to reservoir 5. Port 76 is connected to port 56 of braking'valve 51. When the pressure in working space 83 is high the spool 78 will move to connect port 76 without restriction to port 77 thereby allowing return liquid passing through valve 51 to pass directly to reservoir. When the pressure in working space 83 is low, e.g. about 250 p.s.i. port 77 is closed and there is a restricted connection between ports 75 and 76 allowing a small make up flow of liquid to pass through ports 76 and 56 and through a pair of non-return valves 84 and 85 into either of ports 29 or 32 of the throttle valve 25.

A direction selecting valve 86 is controlled in position by a manual lever 87 having positions corresponding to forward, neutral and reverse. The valve 86 comprises a cylinder 87 having a spool 88 therein comprising three lands 89, 91 and 92. The lands 89 and 91 control ports 93 and 94 which are connected through pipes 95 and 96 to the working spaces 41 and 42 of the throttle valve 25. A port 97 enters the cylinder 87 between the ports 93 and 94 depending on the position of the spool '88, port 97 is connected to either of the ports 93 or 94. A detent flange 98'on the spool 88 is engageable by a pawl 99 controlled by a piston-and-cylinder unit 101. The piston-and-cylinder unit is spring-loaded so that the spool disengages from the flange 98 and is connected through pipes 102 and 103 to respond to the difference in pressures between the working spaces 61 and 62 of the braking valve 51. A small pressure difference only e.g. about 25 p.s.i. is sufficient to urge the pawl 99 inwardly to engage flange 98.

The lever 87 controls the spool 88 through the medium of a caged spring 104.

A master piston-and-cylinder unit 105 is formed by a piston 106 slidable in a cylinder 107 against a spring 110. The piston 106 is urged into cylinder 107 by means of a servo piston 108 slidable within a cylinder 109. Cylinder 109 receives liquid at reduced pressure through pipe 111. A rod 112 extends between the pistons 108 and 106 through a reservoir chamber 113. A priming passage 114 extends from the reservoir chamher 113 into cylinder 107 at a position where it is just opened by piston 106 when in its fully retracted position. A bleed passage 115 extends through piston 108 and rod 112 to open into the reservoir chamber 113, the opening being controlled by a sleeve 116 slidable on rod 112. The position of sleeve 116 is adjusted by means of a foot pedal 117, the sleeve 116 being restored to its right hand position as shown by means of a spring 118 acting on the pedal 117.

A pressure reducing valve 1 19 receives liquid at pressure from the pump delivery connection 6 and by virtue of conventional spring action supplies reduced pressure through restrictor 121 to a port 122 opening into the cylinder 87 of selector valve 86. An internal passage 123 Within the land 92 of the selector valve connects port 122 to reservoir when lever 87 is in the neutral position thus ensuring that when lever 87 is in the neutral position no servo liquid at pressure can be delivered to servo cylinder 109.

The servo motor for adjustment of displacement of the motor 3 is shown at 124. This servo motor is of the differential area piston type comprising a stepped cylinder 125 within which a stepped piston 126 is slidable providing a pair of working spaces 127 and 128 of which the working area on the piston of space 127 is one half the working area on the piston of space 128. A control cylinder 129 within the piston carries the servo valve 131 provided with lands so as to connect working space 128 either through a restricted passage 132 to the working space 127 or to drain through a central passage 133 within the spool valve 131 and passage 134 extending from the end of the piston. A spring 135 within cylinder 129 urges the spool 131 in a downward direction. The piston 126 further defines with its stepped bore a working space 136 connected by pipe 137 with the master cylinder 107. The working space 127 is fed with high pressure liquid from the motor connections 47 and 48 by virtue of two non-return valves 138 and 139 which select the higher pressure from the motor connections to feed to the working space 127.

The motor servo 124 also includes an override valve 141 whose function is to cause the servo motor 124 to move overridingly to a larger motor displacement when the pressure exceeds 3000 p.s.i. The valve 141 is fed with high pressure liquid from the working space 127 of the servo through a pipe 142, such pressure being fed to act on a spool 143 against the compression of a spring 144. If the pressure exceeds a predetermined value, say, 3000 p.s.i., causing the spool 143 to move against the spring 144, the movement of the spool will connect a pipe 145 extending from the working space 128 to a pipe 146 extending to reservoir 5. The control pressure in the space 128 will cause movement of the servo piston to overridingly increase motor displacement until the pressure in working space 127 reduces below the level i.e. 3000 p.s.i., to which the valve 141 responds.

The transmission described is intended for use on a vehicle to drive the propulsion wheels thereof and various functions of the apparatus will now be described.

Forward Propulsion In order to propel the vehicle forwardly, the direction lever 87 is moved to the appropriate forward position moving the spool 88 to the left as seen in the drawing thus connecting the master cylinder 107 through pipe 137, ports 97 and 93 of valve 86 and pipe 95 to the left hand working space 41 of the throttle valve 25. Assume the engine is running at its normal speed. In order to propel the vehicle the pedal 117 is then depressed from position A towards position B. The fact that the engine is running and the pump delivering means that liquid under pressure is supplied through pipe 6 to the pres sure reducing valve 119 and the selection of forward on lever 87 will disconnect the reservoir passage 123 from port 122 thus ensuring the delivery of liquid at reduced pressure to the servo motor 109. Depression of the pedal will move sleeve 116 to cover passage 115 so that pressure is developed against the servo piston 108 to move it to the left moving master piston 106 into cylinder 107 to cut off port 144 and thus to pressurise liquid in the master cylinder. This liquid is transferred through pipe 137 to working space 41 urging the spool 34 to the right, firstly, displacing the locating pawl 45 and then slightly compressing the first spring 43. At this position, the land 36 will uncover the auxiliary port 53 and will open a restricted connection between ports 31 and 29, so that high pressure liquid from the pump through pipe 24 may flow through port 29 into motor connection 47 thus causing the motor to rotate. The dividing valve 7 will receive in its working spaces 17 and 18 the pressures respectively from auxiliary port 53 and from port 31 such forces urging the spool 8 as seen in the drawing to open a flow path from port 12 to port 11 and to throttle the flow from port 12 into port 13. This action will provide flow from the pump delivery into the ports 11 and 13, the proportion between these two flows being dependent on the pressure drop from port 31 to port 29. The adjustment will be such that the pressure drop is maintained at a constant value depending on the load of the spring 21. In other words the flow to the'motor will be in direct proportion to the movement of the throttle spool 34 which in turn is in proportion to the pressure generated in the master cylinder 107.

Return flow from the motor passes along pipe 48 to port 32 of the throttle valve and then into port 33. From port 33 the flow will enter pipe 49 and port 55 of the braking valve 51. The pilot valve 66 will also receive the return pressure from pipe 49 which will be quite low i.e. considerably ,less than the pressure at which the pilot valve 66 will start to vent working space 62. The pressure from the auxiliary port 53 which is the pressure in the pipe 47 carrying flow to the motor, will react on the pilot valve 67 causing full opening thereof so that the working space 62 of braking valve 51 is vented directly to reservoir. Thus the pressure in the return flow of liquid in port 55 can act on the spool 57 urging it to the right against the spring 63 and connecting port 55 to port 56 without restriction other than to produce pressure drop sufficient to urge spool 57 against spring 63 which in this embodiment is about 25 psi. From port 56 the return liquid enters the port 76 of the boost pressure valve 73. The working space 83 of valve 73 will receive liquid at high pressure from pipe 22 urging the spool 78 to the right as seen in the drawing providing a substantially unrestricted connection from port 76 into port 77 and so to reservoir.

Assume now that the driver wishes the vehicle to go faster. He will depress pedal 117 to the B position which increases pressure in the master cylinder to cause movement of the throttle valve spool 34 to the right to the extent that the first spring 43 is fully compressed and engagement is just made to start compressing the spring 44. The driver depresses slightly beyond position B and the pressure developed in the master cylinder 107 as applied to the space 136 of the servo motor 124 will now be sufficient to move the pilot valve 131 against the load of its spring 135, thus connecting high pressure to the working space 128 causing the servo piston 126 to move upwardly as seen in the drawing to reduce motor displacement. The flow rate of liquid fed to the motor is that determined by the throttle valve atthe position where it is just about to start compressing spring 44 and increase in speed is obtained by virtue of the fact that the motor displacement is reduced whilst the liquid flow through it is maintained constant. As the driver pushes the pedal 117 from position B to position C the liquid displaced from the master cylinder 107 will cause displacement of the pilot spool 131 so that the servo piston 126 must follow. At any point if the driver does not move the pedal further the pilot valve 131 will stop moving and accordingly the servo piston 126 will stop moving. Assume now that speed has increased to the extent that the motor is at minimum displacement. This will correspond toposition C for the pedal. Again assume that the driver wishes to go faster, he will then depress the pedal from position C to position D. The servo piston 126 will be on its minimum displacement stop and the pilot valve 131 will also be held against a stop in its cylinder so that no further increase in volume of working space 136 is possible. The further movement of the pedal 117 will therefore increase pressure in the master cylinder to the extent that the pressure increase in theworking space 41 will cause spool 34 to move against compression of both springs 43 and 44. This movement will reduce the restricting effect between ports 31 and 29 and the dividing valve 7 in its attempt to maintain a constant pressure drop between these ports will cause a greater proportion of pump delivery to pass through the throttle valve to the motor. At the position D the whole of the pump delivery will effectively pass through the motor at its minimum displacement position, little or no liquid passing through port 13 of the dividing valve to reservoir.

Pressure Override Control If during propulsion at any instant, the pressure fed to the motor attains a value in excess of a preset maximum, e.g. 3000 p.s.i., the pressure limit valve 141 will respond by connecting the working space 128 of the servo to reservoir. The high pressure in the working space 127 of theservo will then move the servo piston in a motor displacement increasing direction, although the pressure in the working space 136 holds the pilot valve 131 fully depressed. This overriding movement of the piston 126 will cause liquid to be displaced from the working space 136 to increase pressure in both the master cylinder 107 and in the working space 41 (assuming forward propulsion is selected). The increased pressure in the master cylinder 107 will not be felt on the pedal 117 because the master piston 106 is servo operated by virtue of sleeve 116. However, the in creased pressure in the working space 41 will cause further movement of the throttle spool 44 to the right against the joint loading of both springs 43 and 44 to increase the selected flow from pump 2 to the motor 3 as described above. Thus the effect of an excessively high pressure at the motor is to increase motor displacement and at the same time to increase flow rate into the motor. In this way substantiallythe same speed of the vehicle may be retained providing the pump 2 the liquid is maintained at say 3000 p.s.i.

Braking When the driver wishes to reduce speed of the vehicle, he will merely raise the pedal 117 an amount in accordance with the speed reduction desired. The basic action is independent of the actual position of the pedal and is as follows. The raising of the pedal will either reduce the flow rate selected by the throttle drive 25 or increase motor displacement, such operation immediately serving to make the motor rotate at a speed greater than that dictated by the flow rate of liquid to the motor. There will be an instantaneous pressure reversal in the motor connections, the connection 47 carrying liquid to the motor suddenly dropping to a low pressure, and the connection 48 carrying liquid from the motor suddenly increasing to a high pressure. The flow path for the return liquid from pipe 48 is through ports 32 and 33 of the throttle valve, pipe 49, ports 55 and 56 of the braking valve and through non-return valve 84 into the port 29 leading to the flow pipe 47. Thereby a closed circuit is formed for the liquid flowing through the motor and substantial pressure is lost in this circuit in flow between the ports 55 and 56. The pilot valve 67 by virtue of low pressure from pipe 22 is not operative to vent liquid from working space 62 of the braking valve, but the pilot valve 66 which now receives high pressure from the pipe 49 will permit a restricted flow of liquid from the working space 62 in the sense to ensure that a pressure drop of some 2000 p.s.i. will occur when liquid passes from port 55 to port 56. This closed circuit within which liquid flows to and from the motor, will tend to lose liquid due to leakage and therefore liquid must be fed into the circuit to prevent cavitation. This function is performed by the boost valve 73 which on receipt of the comparatively low pressure from pipe 22 during braking, will slightly move the spool 78 to the extent to provide a restricted connection between the pump delivery pipe 6 through port 75 and into port 76 for feeding to the port 56, which is the position in the closed motor circuit having the lowest pressure during braking. This function will ensure that liquid at the pump delivery pressure during braking is fed into the closed motor braking circuit.

Prevention of Selecting Change in Direction Whilst Vehicle in Motion When the vehicle is in motion in a particular direction, the lever 87 will be in the forward or the reverse position in which the reservoir passage 123 is cut off from port 122 and in which the locking piston and cylinder unit 101 is energised from the pressure drop at the braking valve to urge the pawl 99 to engage one or other side of the flange 98. Whilst the vehicle is in motion, the return flow of liquid from the motor through the braking valve, whether during braking or propulsion will produce a small pressure drop between the two working spaces 61 and 62, this pressure drop acting on the piston and cylinder unit 101 against its light spring loading to hold pawl 99 in engagement with flange 98. Only when the vehicle has actually stopped movement and the motor has stopped rotation, does the pressure difference between the working spaces of the braking valve vanish, and at this instant the spring loading will withdraw the pawl 99 allowing selection of the alternative direction. If before the vehicle has stopped motion, the driver attempts to select the alternative direction of motion, he could just succeed by moving lever 87 to cause the reservoir passage 123 to coincide with port 122, thereby removing the pressure from the pipe 111. In this case the master piston 106 will immediately retract completely, and the action will be to cause vehicle braking to bring the vehicle to a complete halt. The pawl 99 will then remove and the alternative direction of propulsion can be selected.

The operations of propulsion and braking have been described above for the forward direction of propulsion in which the lever 87 is set in the forward position. For reverse propulsion, the lever 87 is moved into the reverse position thereby moving the spool 88 to the right as seen in the drawing and connecting the master cylinder 107 through pipe 137, ports 97 and 94, valve 86 and pipe 96 to the right-hand working space 42 of the throttle valve 25. The operations previously described for forward propulsion can then take place for reverse propulsion, the only effective difference being that the throttle valve 25 is now controlled by pressure in the working space 42 so that liquid at pressure delivered by the pump 6 through the flow dividing valve 7 in pipe 24 to the valve 25 will now leave the valve 25 through port 32 and pipe 48 to drive the motor 3 in reverse, The effects described for forward propulsion can take place similarly during reverse propulsion by virtue of the following structural features:

a. The springs 43 and 44 act similarly on the spool valve member 34 for either direction of movement from the neutral position.

b. The non-return valves 84 and permit make-up flow of liquid to pass through the ports 29 or 32 of the throttle valve 25 to the port at the lower pressure irrespectively of the direction of propulsion.

c. Adjustment of motor-displacement by means of servo motor 124 operates in the same manner both for forward and reverse propulsion. d. The non-return valves 138 and 139 act to select the high pressure from the motor connections 47 and 48 irrespectively of the direction of propulsion to supply liquid for operation of the servo-motor 124.

e. The throttle valve 25 includeslands 35 and 37 to arranged that return flow liquid irrespectively of the direction of propulsion is directed through pipe 49 to brake valve 51.

f. Within the throttle valve 25 the land 36 ensures that only one or other of the two ports 52 and 55 is opened at any one time thus ensuring that the signal of pressure loss due to flow through the throttle valve from the pipe 24 is similarly fed to flow-dividing valve 7 to control its operation irrespectively of selection of forward or reverse propulsion.

In accordance with a further feature of the invention, a power transmission comprises, a positivedisplacement pump, a positive-displacement motor, a

reservoir connected to the pump, a supply passage inter-connecting the pump with the motor to carry liquid at pressure from the pump to the motor, a return passage from the motor and a braking valve interconnecting the supply passage, the return passage and the reservoir, the braking valve being arranged to respond to pressure in the supply passage such that for high pressure in the supply passage the motor return is connected without substantial restriction to reservoir and for low pressure in the supply passage the motor return is connected with substantial throttling to the supply passage. The pump may include a variable bypass adapted to controllably select a proportion of pump delivery for flow into a by-pass circuit, the remainder being directed along the supply passage to the motor. A make-up valve may be provided which is arranged to respond to low pressure in the said supply passage to connect the pump delivery to the said supply passage to feed liquid to the supply passage which could otherwise be directed through the by-pass circuit.

In accordance with a further feature of the present invention, a power transmission comprises, a positivedisplacement pump, a positive-displacement motor, valve means for controlling the flow rate of liquid from the pump to the motor and also for controlling the direction of flow to the motor, a direction selector acting on said valve means to select the direction of flow of liquid to the motor and a speed selector operative on said valve means to select the flow rate of liquid to the motor, said speed selector acting on the valve means through the medium of a servo-motor and said direction selector being arranged to cut off servo power to said speed selector when in the neutral position be tween selecting forward and reverse flow for said motor. A latch device may be provided operative in response to rotation of said motor to lock the direction selector against movement to select the opposite direction of rotation.

ln accordance with a further feature of the present invention, a servo control device comprises, a servo piston mounted for movement in a servo cylinder, a hydraulic power supply, a pilot valve mounted within said servo piston and capable of controlling liquid flow from said power source to said servo piston to cause movement thereof in accordance with movement of the pilot valve within the servo piston, a pilot piston-andcylinder device, a master piston-and-cylinder remotely locatable relative to said servo motor, manual means for moving said master piston within its cylinder, a by draulic passage connecting the said master cylinder to said pilot cylinder, the arrangement being that displacement of liquid by the manual control from said master cylinder causes deflection of said pilot piston to operate said pilot valve to cause movement of said servo piston, movement of said servo piston varying the volume of liquid fed by said master cylinder to said pilot cylinder in the sense to permit the pilot valve to move to a neutral position after movement of the servo piston to an amount in proportion to the liquid displaced by said master cylinder.

In accordance with a still further feature of the pres ent invention, a power transmission comprises, a positive-displacement pump, a positive-displacement motor, a double-acting throttle valve forming the connection between pump delivery and the motor, a dividing valve responding to the pressure drop across said throttle valve to controllably divide the pump delivery into flow through said throttle valve and flow in a by-pass circuit in such manner as to maintain constant pressure drop across said throttle valve, said throttle valve including a pair of throttle sections alternatively usable to feed liquid from said pump to said motor in one other direction, piston-and-cylinder means associated with said throttle valve having a pair of opposed working spaces, a master piston-and-cylinder, a manual control for adjusting said master piston-and-cylinder and a reversing valve arranged alternatively to connect the master cylinder to one working space or the other of said throttle valve.

I claim:

1. A hydraulic power transmission comprising a fixed positive-displacement pump, a variable positivedisplacment motor, a first slave piston-and-cylinder unit arranged to adjust motor displacement, a second slave pistonandcy]inder unit arranged to adjustably control a variable by-pass for selectively by-passing a portion of pump delivery; the remainder being fed to the motor, a master piston-and-cylinde unit adjustable by a main control to displace liquid to the slave pistonand-cylinder units and mechanical means operative as liquid is displaced from the master piston-and-cylinder unit to cause movement of the slave units in a desired sequence.

2. A hydraulic power transmission as claimed in claim 1, wherein the variable by-pass comprises a adjustable by the second slave piston-and-cylinder unit and throttle valve carrying liquid flow to the motor and a dividing valve responding to pressure drop across the throttle valve to divide pump delivery to flow partly through the throttle to the motor and partly through a by-pass circuit, the division of pump delivery being in the sense to maintain constant the pressure drop at the throttle.

3. A hydraulic power transmission as claimed in claim 1, wherein the sequence of slave piston-andcylinder unit operation ensured by the mechanical means during increase in motor speed comprises firstly, adjustment of the variable by-pass by the second slave piston-and-cylinder unit to increase flow to the motor, and, secondly, reduction of motor displacement by the first slave piston-and-cylinder unit.

4. A hydraulic power transmission as claimed in claim 3, wherein the sequence of slave piston-andcylinder unit operation ensured by the mechanical means includes, thirdly, the further adjustment of the variable by-pass by the second slave piston-andcylinder unit to further increase flow rate to the motor.

5. A hydraulic power transmission as claimed in claim 4, wherein the second slave piston-and-cylinder unit includes two stages of spring loading, responding to differing ranges of hydraulic pressure from said master cylinder to effect said first and third adjustments.

6. A hdyraulic power transmission as claimed in claim 1, including a pressure responsive means responding to excessive hydralic pressure within the motor to overridingly increase motor'displacement and to react on the first slave piston-and-cylinder unit to increase pressure therein and thereby to cause or tend to cause adjustment of the second slave piston-andcylinder unit to adjust the variable by-pass to increase flow from the pump to the motor.

7. A hydraulic power transmission as claimed in claim 1, including a braking valve connected in the return flow passage from the motor and responding on increase in pressure of the return flow above a predetermined value to exert a throttling effect on the return flow whereby to resist rotation of the motor.

8. A hydraulic power transmission as claimed in claim 7, including a boost valve responsive to low inlet pressure to the motor during braking to feed liquid from the braking valve back to the motor inlet together with liquid from pump delivery in the sense to maintain a predetermined low boost pressure at the motor inlet. 9. A hydraulic power transmission as claimed in claim 2, wherein the adjustable throttle valve includes a pair of throttle sections alternatively selectable, one

in the forward and the other in the reverse position to provide the throttle effect during forward or reverse motor rotation, said throttle valve also acting to select the appropriate throttle section for connection to the dividing valve during forward or reverse motor rotation whereby the dividing valve always responds to the pressure drop at the throttle section carrying liquid from the pump to the motor.

10. A hydraulic power transmission as claimed in claim 9, wherein the adjustable throttle valve includes a pair of opposed slave piston-and-cylinder units, and a direction selecting valve is provided to connect the master cylinder alternatively to one or the other of this pair of slave units to form the second unit depending on the selection of forward or reverse motor rotation by the direction selecting valve.

11. A hydraulic power transmission as claimed in claim 10, including a pawl hydraulically operative in response to motor rotation to lock the direction selecting valve in its selected position.

12. A hydraulic power transmission as claimed in claim 11, including a hydraulic servo-motor for adjusting the master piston-and-cylinder unit in response to movement of the main control member and an on/off valve operable with the direction selecting valve to connect hydraulic pressure liquid for operation of the servo-motor only when the direction selecting valve is in the forward or reverse position.

13. A hydraulic power transmission as claimed in claim 9, including lost motion means for the adjustable throttle valve, providing lost motion between the positions of the throttle valve in which the forward and reverse throttle sections become effective.

14. A hydraulic power transmission as claimed in claim 13, including a spring loaded pawl device providing a predetermined resistance to motion of the throttle valve within its lost motion between the positions wherein the foreward and reverse throttle sections become effective.

15. A hydraulic power transmission as claimed in claim 1, wherein said mechanical means comprises spring loading means for each slave piston-and-cylinder unit. y 

1. A hydraulic power transmission comprising a fixed positivedisplacement pump, a variable positive-displacment motor, a first slave piston-and-cylinder unit arranged to adjust motor displacement, a second slave piston-and-cylinder unit arranged to adjustably control a variable by-pass for selectively by-passing a portion of pump delivery; the remainder being fed to the motor, a master piston-and-cylinde unit adjustable by a main control to displace liquid to the slave piston-and-cylinder units and mechanical means operative as liquid is displaced from the master piston-and-cylinder unit to cause movement of the slave units in a desired sequence.
 2. A hydraulic power transmission as claimed in claim 1, wherein the variable by-pass comprises a adjustable by the second slave piston-and-cylinder unit and throttle valve carrying liquid flow to the motor and a dividing valve responding to pressure drop across the throttle valve to divide pump delivery to flow partly through the throttle to the motor and partly through a by-pass circuit, the division of pump delivery being in the sense to maintain constant the pressure drop at the throttle.
 3. A hydraulic power transmission as claimed in claim 1, wherein the sequence of slave piston-and-cylinder unit operation ensured by the mechanical means during increase in motor speed comprises firstly, adjustment of the variable by-pass by the second slave piston-and-cylinder unit to increase flow to the motor, and, secondly, reduction of motor displacement by the first slave piston-and-cylinder unit.
 4. A hydraulic power transmission as claimed in claim 3, wherein the sequence of slave piston-and-cylinder unit operation ensured by the mechanical means includes, thirdly, the further adjustment of the variable by-pass by the second slave piston-and-cylinder unit to further increase flow rate to the motor.
 5. A hydraulic power transmission as claimed in claim 4, wherein the second slave piston-and-cylinder unit includes two stages of spring loading, responding to differing ranges of hydraulic pressure from said master cylinder to effect said first and third adjustments.
 6. A hdyraulic power transmission as claimed in claim 1, including a pressure responsive means responding to excessive hydralic pressure within the motor to overridingly increase motor displacement and to react on the first slave piston-and-cylinder unit to increase pressure therein and thereby to cause or tend to cause adjustment of the second slave piston-and-cylinder unit to adjust the variable by-pass to increase flow from the pump to the motor.
 7. A hydraulic power transmission as claimed in claim 1, including a braking valve connected in the return fLow passage from the motor and responding on increase in pressure of the return flow above a predetermined value to exert a throttling effect on the return flow whereby to resist rotation of the motor.
 8. A hydraulic power transmission as claimed in claim 7, including a boost valve responsive to low inlet pressure to the motor during braking to feed liquid from the braking valve back to the motor inlet together with liquid from pump delivery in the sense to maintain a predetermined low boost pressure at the motor inlet.
 9. A hydraulic power transmission as claimed in claim 2, wherein the adjustable throttle valve includes a pair of throttle sections alternatively selectable, one in the forward and the other in the reverse position to provide the throttle effect during forward or reverse motor rotation, said throttle valve also acting to select the appropriate throttle section for connection to the dividing valve during forward or reverse motor rotation whereby the dividing valve always responds to the pressure drop at the throttle section carrying liquid from the pump to the motor.
 10. A hydraulic power transmission as claimed in claim 9, wherein the adjustable throttle valve includes a pair of opposed slave piston-and-cylinder units, and a direction selecting valve is provided to connect the master cylinder alternatively to one or the other of this pair of slave units to form the second unit depending on the selection of forward or reverse motor rotation by the direction selecting valve.
 11. A hydraulic power transmission as claimed in claim 10, including a pawl hydraulically operative in response to motor rotation to lock the direction selecting valve in its selected position.
 12. A hydraulic power transmission as claimed in claim 11, including a hydraulic servo-motor for adjusting the master piston-and-cylinder unit in response to movement of the main control member and an on/off valve operable with the direction selecting valve to connect hydraulic pressure liquid for operation of the servo-motor only when the direction selecting valve is in the forward or reverse position.
 13. A hydraulic power transmission as claimed in claim 9, including lost motion means for the adjustable throttle valve, providing lost motion between the positions of the throttle valve in which the forward and reverse throttle sections become effective.
 14. A hydraulic power transmission as claimed in claim 13, including a spring loaded pawl device providing a predetermined resistance to motion of the throttle valve within its lost motion between the positions wherein the foreward and reverse throttle sections become effective.
 15. A hydraulic power transmission as claimed in claim 1, wherein said mechanical means comprises spring loading means for each slave piston-and-cylinder unit. 